In a wavelength division multiplexed (WDM) network each node is capable of routing a large volume of traffic around the network without the need for optoelectronic conversion and processing. Fibers entering and leaving the nodes carry multiple optical channels (wavelengths), and within the node, optical channels are routed either to the outgoing fiber or to local units (receiver or transmitter) for channel dropping, adding, and re-routing. The WDM network requires reliable supervisory technique for channel identification and performance monitoring. In particular, for proper management of these networks, it is essential to monitor optical paths of WDM signals in optical add/drop multiplexing and cross-connect nodes.
Pilot tone (dither tone) monitor technique is being used to supervise an individual channel along the optical signal path in the WDM network. At the transmitter, a low frequency pilot tone (dither tone) in the kHz range is modulated onto the signal, each optical channel being coded by a different pilot tone frequency. The pilot tones are extracted at intermediate nodes by tapping off a small portion of the optical signal power into a monitor module, while the main part of the optical signal is routed to the destination nodes. The tapped optical signal is used for channel power measurements and identification via the dither tones encoded on the channel. In this way the dither tones can be monitored at each intermediate node without optically demultiplexing the optical data signals. The signal channel identification and power level information provided by the dither tones is useful for fault management, such as in detecting missing channels or low power channels, and taking corresponding corrective actions.
The pilot tones are allocated in a frequency band low enough not to interfere with the optical data spectrum, but high enough to avoid low frequency crosstalk within EDFAs (Erbium Doped Fiber Amplifiers). Otherwise, these pilot tones would generate ghost tones (crosstalk) due to the slow dynamic properties of EDFAs in the optical network and the transmission length (i.e., number of EDFAs) of the signals in the network. These ghost tones could not only mislead the performance monitoring system, but also cause errors in the monitor techniques using pilot tones for channels identifications.
In an article by Chung et al., “Effects of stimulated Raman Scattering on Pilot-Tone Based WDM Supervisory Technique”, IEEE Photon. Technical Letter, Volume 12, pp. 731–733, June 2000, an experimental setup was described including eight WDM signals (optical channels) multiplexed into a single fiber into a single WDM signal transmitted over fiber and eight EDFAs. The first EDFA used a dynamic gain control unit to compensate for the slow dynamic properties of EDFAs. After transmission over fiber for some distance, WDM channels were dropped using add/drop multiplexer (ADM) made of two arrayed-waveguide gratings (AWGs), and the optical spectrum of the WDM signal and electrical spectrum of pilot tones were measured. The ghost tones appeared due to the cross gain modulation of EDFAs. Adjustments were made to the input power of the EDFAs; however, the ghost tones were not removed completely but were suppressed at the output of the first EDFA via the gain control unit.
In an article by Hill et al., “A Transport Network Layer Based on Optical Network Elements”, IEEE Journal of Lightwave Technology, Volume 11, No. 5/6, pp. 667–679, May/June 1993, another method for removing ghost tones was described, wherein channel spacing and commercial fiber amplifiers were used to limit ghost tones build up in an optical network. Unfortunately, there are limitations to channel spacing and commercial fiber amplifiers are not reliable in removing ghost tones completely.
Consequently, there is a need in the industry to provide improved method and apparatus for efficiently and reliably removing ghost tones on WDM signals in an optical network.